1. Field of the Invention
The present invention relates to a packet handler, and in particular, to a fixed length ATM packet handler having a redundant architecture for switchcover to a line interface.
An ATM handler for transferring information using fixed length packets (to be called cells herebelow) includes, for example, a plurality of line interface cards 1-1 to 1-n for accommodating transmission paths (incoming and outgoing lines) 7, an ATM switch (SW) 2 for switching an input cell received from either one of the line interface cards 1 over to another one of the line interface cards according to information for routing included in a cell header of the input cell, and a control part 4 connected via a control line 6 to the line interface cards 1 and the ATM switch 2 as shown in FIG. 2.
Each line interface card 1 includes a line interface circuit 15 for each pair of incoming and outgoing lines. The circuit 15 includes a physical layer processing function for conducting a signal process, for example, an opto-electric conversion and vice versa on a physical layer to process signals communicated with the transmission paths 7 and an ATM layer processing function for accomplishing a signal process on an ATM layer, for example, a process of updating a cell header. Although FIG. 2 shows a configuration in which one line interface circuit s provided for each line interface card, it may also possible to mount a plurality of line interface circuits in one line interface card. Namely, one line interface card can accommodate a plurality of transmission paths.
The control part 4 achieves operations such as a control operation for connecting calls, a re-writing operation of a header conversion table in each line interface circuit 15, an operation to supervise the overall operation of the ATM handler including the operation of the ATM switch 2, and an operation to collect various performance information items acquired by the respective line interface circuits 15. The control part 4 is linked with a network management function 5 via a communication line for network management 8 to transmit in response to an order from the network management function 5 supervision/defect information to the network function 5 which is a higher level device of the control part 4.
In such an ATM network requiring high reliability as a public network, the main transmission path system is, for example, duplicated in a redundant configuration for an alternative route at occurrence of a failure as well as for maintenance and inspection of the system.
FIG. 3 shows as an example of the redundant configuration of ATM network a network configuration including a duplicated system of transmission paths (backbone network) 7-b connecting ATM handlers 11 (11A to 11C) to each other.
In relation to the redundant system of backbone network 7-b, each ATM handler 11 includes a duplicated system of line interface cards 1-b accommodating the backbone network 7-b (redundant transmission paths). In this construction, one of the cards 1-b is used as an active system and the remaining one thereof is adopted as a standby system in a selective fashion.
Reference numeral 7-a indicates transmission paths on the subscriber's side. In this example, one line interface card 1-a accommodates a plurality of subscriber lines, for which there are missing the standby system. This also applies to the line interface cards related thereto. However, to guarantee reliability of communication, the redundant configuration may be adopted also for the subscriber lines depending on cases.
FIG. 4 shows the construction of an ATM handler 11 accommodating the transmission paths in the redundant system.
Numeral 1 (1-1 to 1-n) indicate line interface cards each including a line interface circuit 15. In association with the duplicated transmission paths, for a set or pair of two line interface circuits associated with a working path and a protection path, a stream of cells from either one pertinent transmission path is delivered by selectors 9 (9-1 to 9-j) disposed on a selector card 3 to the ATM switch 2. In the subsequent description, of the two transmission paths including working and protection paths, the path selected by the selector 9 is called an active path or system and the remaining one thereof is called a standby path.
An input cell received from a line interface on the active system side is supplied via the selector card 3 to an input port of the ATM switch 2 to be then routed to either one of the output according to the contents of the cell header thereof. The output cell is sent via the selector card 3 to be broadcast to line interface cards of the active and standby systems corresponding to the output port. The cell is thereby transmitted to the respective transmission paths. In a case in which the transmission path is required to be switched over because of, for example, a failure detected in a transmission path on the active system side, a predetermined switchover, or a system maintenance, a system switchover order is notified to the selector 9 according to an order from the network management function 5 or a decision of the control part 4.
In the ATM handler 11, to supervise the statuses of paths and connections duplicated on the respective transmission paths, it is required to collect performance information measured by each line interface circuit 15. The performance information includes, for example, a count value of user cells, a count value of invalid cells, a count value of unassigned cells, and a count value of OAM (Operation and Maintenance) cells respectively measured for each connection measured by an ATM layer processing block of each line interface circuit. Of these performance information items, the count value of user cells (to be called the number of user cells herebelow) for each connection/path is particularly employed as basic data for an accounting operation. Therefore, it should be taken into consideration to prevent an event in which same cell is duplicatedly counted by mistake in the course of system switchover.
FIG. 5 shows a point for collecting the number of user cells in the ATM handler 11 accommodating duplicated transmission paths and non-duplicated ordinary transmission paths.
In FIG. 5, reference numerals 1-1 and 1-2 indicates line interface cards in pair respectively for working and protection paths. The system switchover or change-over is carried out by the selector 9. A numeral 1-3 denotes a line interface card linked with an ordinary transmission path not duplicated. In this diagram, for easy understanding the stream of cells, three line interface cards 1-1 to 1-3 related to an ATM connection 10 are arranged on the right and left sides and the selector card 3 is subdivided into a selector card 3-1 on the duplicated transmission path side and a selector card 3-2 on the ordinary transmission path side.
In the ATM handler 11, the number of user cells is counted for each connection by the line interface circuits on the input and output sides. When the line interface cards are duplicated, the user cells are counted on the line interface circuit for the active path.
Assuming in FIG. 5 that the line interface card 1-1 of the working path is the active system, a point for collecting performance information PM is set to an input side PM (a) 40-a and an output side PM (d) 40-d in the line interface card 1-1 and an input side PM (b) 40-b and an output side PM (c) 40-c in the line interface card 1-3. When a system switchover is conducted by the selector 9, the line interface card 1-2 of the protection path in the standby status becomes active and the points for collecting performance information PM are changed, that is, a point for collecting performance information PM is set to PM (a) 40-a and PM (d) 40-d in the line interface card 1-2.
After the system switchover, it is required that each number of user cells counted by the line interface card 1-1 which has been the active system up to this point is transferred to the point of collecting performance information of the line interface card 1-2 thus set to the active system.
FIG. 6 shows a transfer operation of the number of user cells by the line interface circuit on the input side at system switchover. In the graph of FIG. 6 indicating the lapse of time along an abscissa, flows of ATM cells 50 to be counted are schematically depicted. Reference numerals 51-0 and 51-1 respectively stand for flows of input cells 50 and 50' respectively of the working and protection paths, a reference numeral 51-2 denotes a flow of cells inputted via the selector 9 to the ATM switch 2, and an asterisk indicates cells counted by the interface on the input side.
Assume a case in which when the line interface circuit 15-1 of the working path is in the active status, the selector conducts a system switchover (arrow mark 52) in response to an order from the control part 4 such that the count operation of user cells is changed over to the line interface circuit 15-2 of the protection path at a point of time (arrow mark 55) after a lapse of delay time T. The delay time T is inevitable required when the control part 4 communicates control orders and performance information with a large number of line interface cards according to predetermined protocols.
When there is missing any difference in phase of the stream of cells between the line interface circuits of the working and protection paths and cells are normally supplied to both transmission paths (working and protection paths) in a successive manner even after the system switchover like in a case of a routine maintenance, the line interface circuit 15-1 of the working path can count input cells 50 (fourth and fifth cells) during the delay time of switch notification from when the selector conducts the switchover of the stream of cells to when the counter operation is changed over. Therefore, it is guaranteed that the line interface circuit 15-2 of the protection path count on the input side the input cells 50' (fourth and fifth cells) not counted by the line interface circuit 15-1 of the working path, thereby transferring the count operation from the working path to the protection path.
In other words, the control part 4 acquires the number of user cells up to the fifth cell from the line interface circuit 15-1 of the working path to add after the system switchover the number to the number of user cells including the sixth and subsequent user cells counted by the line interface circuit 15-2 of the protection path. As a result, it is possible to appropriately obtain the number of user cells inputted to the ATM handler 11. Additionally, the selector can receive, after the last input cell 50 (third cell) from the line interface circuit 15-1 of the working path, the succeeding cells 50' fourth cell, fifth cell, etc.) the line interface circuit 15-2 of the protection path without losing any cells. In consequence, the line interface circuit 15-3 on the output side of the ATM switch 2 can correctly count the number of output user cells.
However, in the example shown in FIG. 6, it is assumed that (1) the cells are supplied to the working and protection paths without a phase difference therebetween and (2) cells are continuously inputted to the respective redundant transmission paths also after the selector switches over the system. Consequently, the cell counting operation cannot be appropriately accomplished when these premises are changed.
For example, when a defect or failure takes place in a transmission path in the active status, since the stream of cells 51-0 is interrupted in the line interface circuit 15-1 of the working path at a point of time 52 when the selector conducts the switchover, it is impossible for the line interface circuit 15-1 of the working path to count cells 50 (fourth and fifth cells) inputted from the line interface circuit 15-2 of the protection path to the selector 9 within the switchover delay time T.
In addition, when the delay time of cell transmission varies between the duplicated transmission paths, a phase difference appears for the input cell between the working and protection paths in the line interface circuit on the input side of the ATM handler. For example, as can be seen from FIG. 7, assume that the redundant transmission paths 7 establish connections between an ATM handler 11-1 at Osaka and an ATM handler 11-2 at Tokyo, a transmission path 13-a to be used as 6, protection path is provided via systems for data transmission 12 and 12' installed respectively in kanazawa and Maebashi, and another transmission path 13-0 to be adopted as a working path is disposed along Tokaido. In this case, due to the difference between the respective transmission paths and that between devices at nodes, there appears between the transmission paths a delay difference which cannot be ignored.
Assume that the difference in transmission delay is represented as Td and a phase difference of at least one cell (two cells in this case) takes place in the stream of input cells, for example, in the line interface circuits 15-1 and 15-2 respectively of the working and protection paths as shown in FIG. 8. When the selector 9 is changed over at time 52 in a manner similar to that used in FIG. 6, cells 50 (fourth and fifth cells) which are counted by the line interface circuit 15-1 of the working path and which are already inputted to the ATM switch 2 arrive at the line interface circuit 15-2 of the protection path with a delay. This results in a difficulty that these cells 50 are duplicated supplied to the ATM switch 2. Consequently, the number of cells counted on the output side of the ATM switch 2 is larger than the actual number of cells and hence an error occurs in the accounting process.
In addition, even when the system is switched over in a status free of the defect on the transmission path, the continuity with respect to numbers assigned to cells cannot be guaranteed between the last cell 50 (seventh cell) counted on the input side by the circuit 15-1 of the working path and the first cell 50' (sixth cell) counted by the circuit 15-2 of the protection path. Therefore, even when the control part 4 adds to each other the count values respectively of the circuits 15-1 and 15-2 of the working and protection paths, it is impossible to obtain the correct number of user cells.